This invention relates to improvements of piping passing through a nuclear reactor containment vessel.
Generally, in a nuclear power plant, high temperature and high pressure steam generated through thermal energy produced in a nuclear reactor is led from the containment vessel of the reactor to the outside to rotate turbines to generate electricity. After rotating the turbines, steam is condensed into water which is returned to the nuclear reactor in the containment vessel via a feedwater pipe. Besides the main steam pipe for feeding steam to the turbines and the feedwater pipe for returning water to the reactor, pipes for transporting steam and water, for example, a main steam drain line for releasing drain of main steam, nuclear core spray pipes for cooling the reactor core, pipes of system for removing residual heat for cooling the reactor are passed through the containment vessel. Moreover, pipes for transporting gas, such as drywell ventilating pipes for performing ventilation of the containment vessel and pipes of an off-gas system for treating gases are also passed through.
As aforesaid, a multiplicity of pipes through which steam, water and gas are transported are passed through the containment vessel, and each of these pipes is formed with shut-off valves mounted on inside and outside of the wall of the containment vessel. For example, Japanese Patent Application Laid-Open No. 36490/79 discloses the piping passing through the containment vessel and the valves in a nuclear reactor.
The main steam pipe of a boiling-water reactor (BWR) shown in FIG. 1 will now be described.
Water changes into high temperature and high pressure steam due to heat produced by nuclear reactions taking place in a pressure vessel 1 of the nuclear reactor. Steam thus generated usually has a temperature of about 270.degree. C. and a pressure of about 70 kg/cm.sup.2. Steam generated within the pressure vessel 1 is led from a containment vessel 3 of the nuclear reactor via a main steam pipe 2 extending through a wall of the containment vessel 3 to the outside thereof and then to a turbine shed, not shown, where steam rotates turbines to actuate a generator to generate electricity. After being released from the turbines, steam changes back to water which is returned to the pressure vessel 1 in the containment vessel 3.
The containment vessel 3 is formed of steel and of a substantially truncated conical configuration. It has a diameter of about 25-29 m at the bottom and is enclosed by a concrete shell 6 at the outside.
The pressure in the containment vessel 3 is usually at the atmospheric pressure level. However, the containment vessel 3 is designed such that its shell is capable of withstanding an internal pressure equivalent to several atmospheric pressure supposing that an accident happens involving a guillotine rupture of pipes within the containment vessel and high temperature and high pressure steam gushes out of the pipes.
In the case of an advanced boiling-water reactor (ABWR), the containment vessel 3 of the reactor is formed of concrete as shown in FIG. 2 and designed such that an internal pressure is borne by its concrete wall. The concrete wall has a lining of steel plates.
In both the BWR and ABWR, the containment vessel 3 has a function to confine the radioactive materials and to prevent them from being scattered in atmosphere when accidents such as breakage of pressure vessel or piping rupture occur. Thus, the pipes passing through the containment vessel are each provided with valves mounted on the inside and the outside of the containment vessel. Such valves are provided to the main steam pipe as well, which will be described by referring to FIG. 1.
The main steam pipe 2 is usually made of carbon steel and has a diameter in the range between 400 and 700 mm and a wall thickness in the range between 20 and 40 mm. One nuclear power plant generally has four systems of main steam pipe. Each system of main steam piping 2 is provided with an inner main steam shut-off valve 4 and an outer main steam shut-off valve 5 mounted near an inner wall surface and an outer wall surface of the containment vessel 3 respectively as double safety means. The main steam shut-off valves 4 and 5 are closed when steam leaks or other trouble occurs during the operation of the reactor.
When steam leaks occur in the main steam pipe between steam nozzles of the pressure vessel 1 and the inner main steam shut-off valve 4, for example, the inner main steam shut-off valve 4 is closed to cut off the supply of steam to the turbine system while allowing steam to spread within the containment vessel 3. The closure of the inner main steam shut-off valve 4 results in the radioactive steam being confined within the containment vessel 3.
Also, when steam leaks occur in the main steam pipe between the outer main steam shut-off valve 5 and the turbines, the outer main steam shut-off valve 5 is closed and the supply of steam to the turbines is interrupted, thereby minimizing the leaks of steam.
When steam leaks occur in the main steam pipe between the inner main steam shut-off valve 4 and outer main steam shut-off valve 5, the two shut-off valves 4 and 5 are closed to minimize the steam leaks.
Between the two shut-off valves 4 and 5, the main steam pipe 2 extends through an aperture formed in the wall of the containment vessel 3 and is secured to the wall. The containment vessel 3 being used at room temperature, loads are applied by thermal stresses to the portion of the main steam pipe that is secured to the wall of the containment vessel 3, so that the main steam pipe 2 is mounted to the wall of the containment vessel 3 by utilizing highly advanced technology. Yet, it is inevitable that crack formation might occur in the main steam pipe 2 in a portion thereof between the outer wall surface of the containment vessel and a portion of the main steam pipe 2 including the outer main steam shutoff valve 5, thereby causing steam to leak. It is for the purpose of preventing this steam leak that the inner main steam shut-off valve 4 is provided.
In the containment vessel of the prior art described hereinabove, the main steam pipe 2 has the problem that in the event of steam leaks occuring between the outer wall surface of the containment vessel 3 and the portion of the main steam pipe 2 including the outer main steam shut-off valve 5, steam would be released into the atmosphere outside the containment vessel 3 during the period of time from the occurrence of the accident to the closure of the inner main steam shut-off valve 4 and the area of radioactive contamination is enlarged.